Method of processing a workpiece in a plasma reactor with independent wafer edge process gas injection

ABSTRACT

The disclosure concerns a method of processing a workpiece or in a plasma reactor chamber, using independent gas injection at the wafer edge.

TECHNICAL FIELD

The disclosure concerns a plasma reactor chamber for processing aworkpiece such as a semiconductor wafer to produce integrated circuits.Specifically, the disclosure concerns independent process gas injectionat the ceiling and at the wafer edge in such a reactor chamber.

BACKGROUND

In a plasma reactor chamber for etching silicon or polysilicon thinfilms on a semiconductor wafer, a uniform distribution of etch rateacross the wafer is needed. Non-uniform distribution of etch rate acrossthe wafer is indicated by non-uniformity in critical dimension (CD). Thecritical dimension may be a width of a typical line in the thin filmcircuit pattern. The CD is less in areas on the wafer surfaceexperiencing a higher etch rate and greater in areas of lower etch rate.

In silicon etch chambers in which the process gas is injected from theceiling, it has been found that the CD is very small at the wafer edgecompared to other areas on the wafer surface. The effect of a small CDis typically confined to the outer or peripheral 1% of the wafersurface. This problem was not solved using conventional techniques.Specifically, etch uniformity can be improved by dividing the gasdistribution into independent inner and outer gas injection zones at theceiling and maximizing uniformity by adjusting the gas flow rates to theinner and outer zones. However, adjustment of the inner and outer gasinjection zone flow rates does not solve the problem of small CD at theouter 1% of the wafer surface. Specifically, adjustment of the inner andouter gas injection zone flow rates at the ceiling can produce fairlyuniform CD across the wafer, with the exception of a region at the waferedge whose width is about 1% of the wafer diameter.

Therefore, there is need to independently control the CD at the outer 1%of the wafer edge without detracting from etch rate distributionuniformity achieved for the other areas of the wafer.

SUMMARY

A method is provided for processing a workpiece in a plasma reactor. Themethod comprises placing the workpiece on a workpiece support in chamberof the plasma reactor and introducing a first process gas through aworkpiece support process gas injector adjacent and surrounding theperipheral edge of the workpiece. The method further comprises couplingplasma RF source power into the plasma reactor to generate a plasma inthe plasma reactor chamber. In one embodiment, the method may furthercomprise introducing a second process gas into the chamber through aceiling process gas disperser located at a ceiling of the chamber thatoverlies the workpiece support. The gas flow rate through the workpiecesupport process gas injector is controlled independently of the gas flowrate through the ceiling process gas disperser.

In one embodiment, the ceiling process gas disperser comprises an outergas disperser and an inner process gas disperser. The method furthercomprises adjusting gas flow rates through the inner and outer processgas dispersers to optimize process uniformity over a major portion ofthe workpiece. The method may further comprise adjusting process gasflow rate through the workpiece support process gas injector to optimizeprocessing at a peripheral zone of the workpiece.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited embodiments of theinvention are attained and can be understood in detail, a moreparticular description of the invention, briefly summarized above, maybe had by reference to the embodiments thereof which are illustrated inthe appended drawings. It is to be noted, however, that the appendeddrawings illustrate only typical embodiments of this invention and aretherefore not to be considered limiting of its scope, for the inventionmay admit to other equally effective embodiments.

FIG. 1 depicts a plasma reactor in accordance with one embodiment.

FIG. 2 illustrates internal structural features of a cathode liner ofthe reactor of FIG. 1.

FIG. 3 is a cross-sectional view taken along lines 3-3 of FIG. 2.

FIG. 4 is a cross-sectional view taken along lines 4-4 of FIG. 2.

FIG. 5 is a detailed view of a portion of the process rings and cathodeliner of one embodiment.

FIG. 6 is a side view corresponding to FIG. 5.

FIG. 7 is a graph depicting radial distribution of SiCl₂ in the reactorof FIG. 1 with and without gas flow through the wafer edge injectorslot.

FIG. 8 illustrates a method in accordance with one embodiment.

FIG. 9 illustrates a method in accordance with another embodiment.

To facilitate understanding, identical reference numerals have beenused, where possible, to designate identical elements that are common tothe figures. The drawings in the figures are all schematic and not toscale.

DETAILED DESCRIPTION

Referring to FIG. 1, a plasma reactor includes a vacuum chamber 100enclosed by a cylindrical side wall 108, a ceiling 110 and a floor 115.A wafer support 125 supports a semiconductor wafer 130 during waferprocessing. The wafer support 125 includes a cathode electrode 135 thatalso serves as an electrostatic chuck (ESC) electrode. The support 125includes an insulating layer 137 separating the electrode 135 from thewafer 130 and an insulating layer 139 separating the electrode 135 fromunderlying components of the wafer support 125. The upper insulatinglayer 137 has a top wafer-supporting surface 137 a. The reactor furtherincludes an inductively coupled source power applicator or coil antenna140 overlying the ceiling 110. An RF plasma source power generator 145is coupled to the coil antenna 140 through an RF impedance match 150. AnRF plasma bias power generator 155 is coupled to the cathode electrode135 through an RF impedance match 160. A D.C. chucking voltage supply161 is connected through a control switch 162 to the ESC electrode 135.An isolation capacitor 163 blocks D.C. current from the supply 161 fromthe RF bias power generator 155.

Process gas is delivered into the chamber interior by a gas distributioninjector 165 on the ceiling 110. The injector 165 consists of an innerzone injector 170 and an outer zone injector 175. Each one of the innerzone injector 170 and the outer zone injector 175 may be implementedwith plural injection holes or, alternatively, as a slit. The inner zoneinjector 170 is oriented to direct process gas toward a center region ofthe chamber. The outer zone injector 175 is oriented to direct processgas toward a peripheral region of the chamber. The inner zone injector170 is coupled through a valve 180 to a gas distribution panel 185. Theouter zone injector 175 is coupled through a valve 190 to the gasdistribution panel 185. Different process gas supplies 101, 102, 103,104, 105 supply different process gases to the gas distribution panel185. As indicated in the drawing of FIG. 1, in one embodiment, each gassupply may be connected separately to different ones of the inner andouter valves 180, 190 through independent valves 195. In the embodimentof FIG. 1, the gas supply 101 contains a fluoro-hydrocarbon gas such asCH₂F₂ or CHF₃, the gas supply 102 contains hydrogen bromide gas, the gassupply 103 contains chlorine gas, the gas supply 104 contains argon gasand the gas supply 105 contains oxygen gas. The gases referred to hereare examples. Any suitable process gas may be used.

The wafer support 125 is surrounded by a ring-shaped cathode liner 200.The cathode liner 200 may be formed of a process-compatible materialsuch as quartz, for example. A process ring 205 covers the top of thecathode liner 200 and covers a peripheral portion of the wafer supportsurface 137 a. The process ring 205 is formed of a process-compatiblematerial such as quartz. The wafer support 125 may contain materialssuch as metal that are incompatible with plasma processing, and liner200 and the ring 205 isolate the wafer support 125 from the plasma. Theradially inner edge 205 a of the process ring 205 is adjacent the edgeof the wafer 130. In one embodiment, the process ring may provideimproved distribution of RF electrical fields.

A silicon or polysilicon etch process employs silicon etch gases such asHBr and Cl₂ to etch silicon material and employs a polymerizing speciessuch as CH₂F₂ or CHF₃ to improve etch profile. The polymer deposits onsidewalls of deep aspect ratio openings in a polymer deposition reactionthat competes with the etch reaction.

The reactor of FIG. 1 can have a problem of poor critical dimension (CD)control at the wafer edge. Typically, the CD is the width a selectedline in the circuit pattern. The CD tends to be smaller at the waferedge than elsewhere on the wafer 130. The problem of a small CD tends tooccur in an annular zone at the edge of the wafer 130 whose width(extending inwardly from the wafer edge) is about 1% of the waferdiameter. (This narrow zone will hereafter be referred to as the waferedge zone 130 a shown in FIG. 5, which is discussed later in thisspecification.) Over the remainder of the wafer 130, such problems areminimized or prevented by adjusting the valves 180 and 190 to obtain anoptimum ratio of process gas flow rates to the inner and outer gasceiling injectors 170, 175. However, such an optimum adjustment does notsolve the problem of poor CD control at the wafer edge zone 130 a. Thesmall CD at the wafer edge zone 130 a is indicative of a higher etchrate at the wafer edge zone than elsewhere.

We have discovered that the gas flow velocity over the wafer edge zone130 a is extremely low relative to gas flow velocity over most otherportions of the wafer. For instance, in certain applications, while gasflow velocity over the majority of the wafer surface is between about 10and 20 meters per second, gas flow over the wafer edge zone approacheszero. If gas flow over the wafer edge zone is thus stagnant, then thegas residency time over the wafer edge zone is extremely high, yieldingcorrespondingly high dissociation of the process gas species. Such highdissociation can increase the population at the wafer edge zone ofhighly reactive species. Such highly reactive species may includeradicals or neutrals that either (a) etch extremely fast or (b) inhibitpolymer deposition. A highly reactive etch species generated by suchdissociation may include atomic HBr and/or atomic Cl₂, for example. Theresult is a higher etch rate and a correspondingly smaller CD.

In one embodiment, a new gas is injected at the wafer edge to addressthe non-uniform etch rate at the wafer edge. The new gas may be an inertgas such as argon, for example. In one embodiment, the injection of thenew gas increases the gas flow velocity over the wafer edge zone anddecreases process gas residency time over the wafer edge zone. Thedecrease in residency time reduces the population over the wafer edgezone of highly reactive species such as radicals or neutrals. Thevelocity or flow rate at which the new gas is injected at the wafer edgecan be sufficiently low to avoid influencing the etch rate beyond thenarrow wafer edge zone. Typically, the wafer edge zone is about 3 mmwide.

In one embodiment, a polymerizing gas is injected at the wafer edge toaddress the non-uniform etch rate at the wafer edge. The polymerizinggas may be CH₂F₂ or CHF₃, for example. The addition of the polymerizingspecies increases the polymer deposition rate the wafer edge zone, whichdecreases the etch rate. The velocity or flow rate at which thepolymerizing species gas is injected at the wafer edge can besufficiently low to avoid influencing the etch rate beyond the narrowwafer edge zone. Typically, the wafer edge zone is about 3 mm wide.

In one embodiment, the process ring 205 is divided into an upper processring 210 and a lower process ring 212, leaving a narrow circular slit220 between them facing (almost touching) the edge of the wafer 130. Thecircular slit 220 is separated from the edge of the wafer by a verysmall distance in the range of 0.6 mm to 3 mm, e.g., about 1% of thewafer diameter. A desired gas (such as an inert gas or a polymerizingspecies gas) is supplied so as to be ejected from the circular slit 220radially inwardly and directly at the wafer edge. This new gas orpolymerizing species gas may be supplied from the gas distribution panel185.

In one embodiment, an annular gas plenum 225 is provided at the bottomof the cathode liner 200. A cathode gas flow control valve 227 controlsgas flow from the gas distribution panel 185 to the plenum 225 through aconduit 229. Gas is conducted from the plenum 225 to the circular slit220 at the wafer edge by vertical passages 240 inside the cathode liner200.

FIG. 2 illustrates an exemplary internal structure of the cathode liner200. The cathode liner 200 was described with reference to FIG. 1 asbeing formed of an insulator such as quartz. In the embodiment of FIG.2, the cathode liner 200 is formed of metal, and, as shown in FIG. 5, aquartz liner 126 separates the metal cathode liner 200 from the wafersupport 125. The cathode liner 200 includes a cylindrical wall 201having an annular top surface 201 a. An annular base 215 supports thecylindrical wall 201. A shoulder 235 extends in the radially outwarddirection from the base 215 and houses a gas supply inlet 230. Theplenum 225 shown in FIG. 1 is formed within the cathode ring annularbase 215 of FIG. 2, as depicted in the cross-sectional view of FIG. 3.An internal channel 232 extends radially through the shoulder 235 and iscoupled at one end to the gas supply inlet 230 and is coupled at anopposite end to the plenum 225, as depicted in the cross-sectional viewof FIG. 4. As shown in FIG. 2, the vertical passages 240 extend axiallythrough the cylindrical wall 201 and are spaced azimuthally around thecylindrical wall 201. The bottom end of each vertical passage 240 iscoupled to the plenum 225 and the top end of each vertical passage 240opens at the annular top surface 201 a of the cylindrical wall 201. Inone embodiment, the cylindrical wall 201 is about 0.25 inch thick, andeach of the vertical passages 240 is an axial 0.05 inch hole within thecylindrical wall 201.

In the embodiment of FIG. 1, the cylindrical wall 201 supports the lowerprocess ring 212 and the upper process ring 210 is supported on thelower process ring 215.

As shown in FIG. 5, the interior quartz liner 126 surrounds theworkpiece support 125 and is surrounded by the cathode liner cylindricalwall 201. As shown in FIG. 5, the interior liner 126 supports the lowerprocess ring 212, while the cathode liner cylindrical wall 201 supportsthe upper process ring 210. An annular gas feed chamber 260 is boundedby the cylindrical wall top surface 201 a, the upper process ring andthe lower process ring 212. An annular feed passage 262 is formed as agap between the upper and lower process rings 210 and 212. An outerannular protrusion 210 a in the bottom surface of the upper process ring210 faces an outer annular recess 212 a in the top surface of the lowerprocess ring 212. An inner annular recess 210 b is provided in thebottom surface of the upper process ring 210. The inner annular recess210 b faces a raised shoulder 212 b of the lower process ring 212 toform the gas injection slit 220. The protrusion 210 a, the recess 212 a,the recess 210 b and the shoulder 212 b provide the feed passage 262with a meandering path, as shown in FIG. 5. Gas supplied through thevalve 227 of FIG. 1 flows to the cathode or wafer support 125 and entersthe inlet 230 shown in FIG. 4, and then flows through the internalchannel 232 to the plenum 225. From the plenum 225, the gas flows upthrough the vertical channels 240 into the feed chamber 260 of FIG. 5,and then flows through the feed passage 262 into the injection slit 220.

As shown in the side view of FIG. 6, the end or exit port of theinjection slit 220 is within a very short distance D of the edge of thewafer 130, where D is on the order of between 0.6 mm to 3 mm. Given sucha short distance, the effect of gas flow from the injection slit 220 maybe highly localized so as to not affect processing beyond the 3 mm-widewafer edge zone 130 a. Such localization may be realized by establishinga very low gas flow rate within the injection slit 220. For example, thegas flow rate through the valve 227 (to the wafer edge injection slit220) may be between 1% and 10% of the gas flow rate through the valves180 and 190. In this way, the gas flowing out of the injection slit 220affects processing (e.g., etch rate) only in the narrow wafer edge zone130 a, without affecting processing on the remaining portion of thewafer 130.

FIG. 7 is a graph depicting the density of SiCl₂ over the wafer surfaceas a function of radial position in a process in which a polymerizinggas, such as CH₂F₂ or CHF₃ is introduced through the wafer edgeinjection slit 220 of FIGS. 1-6, while an etch process gas such as HBrand Cl₂ is introduced through the ceiling injectors 170, 175. Thedensity of SiCl₂ is an indicator of the degree of polymerization in sucha process. The graph in FIG. 7 shows that, in the absence of any gasflow from the injection slit 220, polymerization is relatively depressedat the wafer edge (curve A). With the polymerizing gas being suppliedthrough the injection slit 220, the degree of polymerization at thewafer edge increases significantly (curve B). The polymerization gasflow through the wafer edge injection slit 220 is limited to a low rate.This limitation of the injection slit flow rate confines the increase inpolymerization to the outer 1% of the wafer diameter, the wafer edgezone. In one example, the etch process gas flow rate through the ceilinginjector nozzles 170, 175 was about 150 sccm while the polymerizationgas flow through the wafer edge injector slot 220 was about 5 sccm.

FIG. 8 illustrates an exemplary method of operating the plasma reactorof FIGS. 1-6 so as to increase CD in the wafer edge zone. A siliconetchant species gas, such as HBr and Cl₂, is injected through the innerzone ceiling injector 170 at a first gas flow rate (block 400 of FIG.8), and through the outer zone ceiling injector 175 at a second gas flowrate (block 405 of FIG. 8). Gas flow through the inner and outer zoneceiling injectors 170, 175 is sufficient to attain a desired averageetch rate across the wafer surface. Etch rate distribution is adjustedover all but the peripheral 1% of the wafer surface by independentlyadjusting the gas flow rates through the inner and outer ceilinginjectors 170, 175 until etch rate distribution uniformity is optimized(block 410 of FIG. 8). This typically leaves the etch rate too high (orthe CD too low) in the wafer edge zone or the outer 1% of the wafersurface. Etch rate is adjusted downwardly (or CD is adjusted upwardly)in the wafer edge zone by reducing gas residency time over the waferedge zone (exclusively) in order to reduce dissociation over the waferedge zone. In one embodiment, reducing the gas residency time over thewafer edge zone is done by flowing through the wafer edge injection slit220 a suitable gas, such as an inert gas or oxygen, to stir up gas flowover the wafer edge (block 415 of FIG. 8). The increase in gas flow, ordecrease in gas residency time, is confined to the wafer edge zone bylimiting the gas flow rate through the wafer edge injector slit to asmall flow rate. This small flow rate is chosen to attain the mostuniform CD distribution, which may be affected by the choice of processgas species, and may be in the range of 1-20 sccm, for example.

FIG. 9 illustrates another exemplary method of operating the plasmareactor of FIGS. 1-6 so as to increase CD in the wafer edge zone. Asilicon etchant species gas, such as HBr and Cl₂, is injected throughthe inner zone ceiling injector 170 at a first gas flow rate (block 420of FIG. 9), and through the outer zone ceiling injector 175 at a secondgas flow rate (block 425 of FIG. 9). Gas flow through the inner andouter zone ceiling injectors 170, 175 is sufficient to attain a desiredaverage etch rate across the wafer surface. Etch rate distribution isadjusted over all but the peripheral 1% of the wafer surface byindependently adjusting the gas flow rates through the inner and outerceiling injectors 170, 175 until etch rate distribution uniformity isoptimized (block 430 of FIG. 9). This typically leaves the etch rate toohigh (or the CD too low) in the wafer edge zone or the outer 1% of thewafer surface. Etch rate is adjusted downwardly (or CD is adjustedupwardly) in the wafer edge zone by increasing polymerization over thewafer edge zone (exclusively) in order to reduce etch rate over thewafer edge zone. In one embodiment, increasing polymerization over thewafer edge zone is done by flowing through the wafer edge injection slit220 a polymerization gas, such as CH₂F₂ or CHF₃ (block 435 of FIG. 9).The resulting increase in polymer deposition rate increases the CD. Thisincrease is confined to the wafer edge zone by limiting the gas flowrate through the wafer edge injector slit to a small flow rate. Thissmall flow rate is chosen to attain the most uniform CD distribution,which may be affected by the choice of process gas species, and may bein the range of 1-20 sccm, for example.

In either one of the methods of FIG. 8 or 9, further optimization isachieved by adjusting the gas flow rates through the ceiling injectors170 and 175 and/or adjusting the gas flow rates through the wafer edgeslit 220. For example, the etchant gas flow through the ceilinginjectors 170, 175 may be reduced while increasing inert orpolymerization gas flow through the wafer edge slit 220 to furtherincrease CD at the wafer edge zone. However, the flow rate through thewafer edge slit can be sufficiently low in order to confine the effectsto the wafer edge zone. However, the etchant gas flow rate through theceiling injectors 170, 175 may be decreased as low as desired (e.g., tozero). And conversely, the etchant gas flow through the ceilinginjectors 170, 175 may be increased while decreasing inert orpolymerization gas flow through the wafer edge slit 220 to decrease CDat the wafer edge zone.

While the invention has been described with reference to embodiments inwhich a selected gas is injected next to wafer edge through a continuousslit injector, the injector at the wafer edge may assume other forms,such as an array or succession of many gas injection orifices around thewafer edge.

While the foregoing is directed to embodiments of the invention, otherand further embodiments of the invention may be devised withoutdeparting from the basic scope thereof, and the scope thereof isdetermined by the claims that follow.

1. A method of performing a plasma etch process on a workpiece surfaceof a workpiece comprising a surface material to be etched, comprising:placing the workpiece on a workpiece support in a chamber of the plasmareactor; introducing through a ceiling gas disperser a first process gascomprising an etchant species capable of causing etching of said surfacematerial in a plasma, said ceiling gas disperser comprising mutuallyconcentric inner and outer gas dispersers; generating a plasma in saidchamber by coupling RF power into said chamber; controlling etch ratedistribution across said workpiece surface by adjusting respective flowrates of said first process gas through respective ones of said innerand outer gas dispersers; reducing etch rate at an edge periphery ofsaid workpiece surface relative to etch rate across a remainder of saidworkpiece surface by injecting a polymerizing gas through a workpiecesupport gas disperser provided on said workpiece support and facing saidedge periphery, said workpiece support gas disperser defining apolymerizing gas injection pattern directed toward said edge peripheryof said workpiece surface and confined generally to said edge periphery.2. The method of claim 1 wherein said edge periphery is limited to onthe order of 1% of an outer portion of a diameter of said workpiecesurface.
 3. The method of claim 1 wherein injecting a polymerizing gasthrough a workpiece support gas disperser comprises supplying saidpolymerizing gas through gas flow channels inside a portion of saidworkpiece support.
 4. The method of claim 3 wherein injecting apolymerizing gas further comprises conducting polymerizing gas receivedfrom said gas flow channels through a process ring on said workpiecesupport surrounding said edge periphery of said workpiece surface. 5.The method of claim 2 further comprising limiting gas flow rate throughsaid workpiece support gas disperser to confine the effect of thepolymerizing gas to edge periphery of said workpiece surface.
 6. Themethod of claim 1 wherein said surface material to be etched comprisesone of silicon or polysilicon, said first process gas comprises one ofHBr or Cl₂, and said polymerizing gas comprises one of CH₂F₂ or CHF₃. 7.The method of claim 4 wherein said injecting a polymerizing gascomprises injecting said polymerizing gas through a slit injectorsurrounding said edge periphery of said workpiece and facing said edgeperiphery and supported on said process ring.
 8. The method of claim 7wherein said slit injector comprises a thin opening extendingcircumferentially around said edge periphery of said workpiece surfaceand extending from a lower boundary coplanar with said workpiece surfaceto an upper boundary above said workpiece surface.